1. Field of the Invention
This invention relates generally to the casting of metal, and more particularly, this invention relates to the casting of molten metal onto a planar area of a continuous moving casting surface such as a continuous moving substrate.
2. Background Information
The metals industry has been developing a process and apparatus for producing an as-cast product that needs little or no additional processing such as hot rolling to reduce it to strip form. One such process that has arisen as a result of this development is the single belt casting process. In such a process molten metal is caused to flow onto a moving horizontal surface such as a continuous belt whereupon it solidifies as it moves horizontally along with the belt. The solid strip of metal is removed from the continuous belt for further processing as desired.
In conventional casting utilizing a horizontal moving mold such as a belt, cooling has typically been provided to the molten metal on the belt from the bottom side. That is, the cooling has been provided through the belt against the bottom side of the metal against the belt. In so doing, it has been noticed that with such type of cooling, porosity may occur in the middle of the cross-section of the strip. This is thought to be due to the fact that even though cooling is directed at the bottom side of the strip, the atmosphere serves to cool the top of the molten metal. Thus, there is directional solidification from both the top and the bottom of the strip of metal as it solidifies. This porosity in the middle of the cross-sectional area of the strip is undesirable in that it cannot be machined or easily removed. In addition, attempts to provide unidirectional cooling from the bottom by rapidly cooling the molten strip from underneath is complicated by the fact that air gaps form between the strip and the substrate which acts as an insulator.
U.S. Pat. No. 4,789,022 discloses a process for casting metal ribbon having unidirectional solidification. However, in that patent, it is required to preheat the substrate to a temperature above the melting point of the molten metal. Additionally, the process disclosed therein is directed to the casting of molten ribbon, not strip as contemplated by the present invention.